Deionization apparatuses for producing deionized water are known in which deionization is performed by passing water to be treated through ion exchangers. In such production apparatuses, there is a need to regenerate ion exchange groups in the ion exchangers with chemicals (alkali or acid) when the deionization performance is reduced due to saturation of the ion exchange groups. More specifically, it is necessary to replace anions and cations adsorbed to the ion exchange groups with H+ derived from an acid and OH− derived from an alkali, respectively. In recent years, electrodeionization apparatuses for producing deionized water that do not need regeneration with chemicals have been developed and put into practical use to overcome the operational drawback described above.
An electrodeionization apparatus for producing deionized water is an apparatus using a combination of electrophoresis and electrodialysis. The basic construction of an ordinary electrodeionization apparatus for producing deionized water is as described below. That is, the electrodeionization apparatus for producing deionized water has a deionization chamber, a pair of concentration chambers placed on opposite sides of the deionization chamber, an anode chamber placed outside one of the concentration chambers, and a cathode chamber placed outside the other concentration chamber. The deionization chamber has an anion exchange membrane and a cation exchange membrane opposite to each other and an ion exchanger (an anion exchanger and/or a cation exchanger) filled between these exchange membranes. In the following description, an electrodeionization apparatus for producing deionized water is sometimes referred to simply as “deionized water producing apparatus”.
To produce deionized water with a deionized water producing apparatus of the above-described construction, water to be treated is passed through the deionization chamber in a state where a direct current voltage is applied between electrodes respectively provided in the anode and cathode chambers. In the deionization chamber, anion components (e.g., Cl−, CO32—, HCO3− and SiO2) are captured by an anion exchanger and cation components (e.g., Na+, Ca2+ and Mg2+) are captured by a cation exchanger. Simultaneously, water-splitting reaction occurs at the interface between the anion exchanger and the cation exchanger in the deionization chamber, thereby generating hydrogen ions and hydroxide ions (2H2O→H++OH). The ion components captured by the ion exchangers are liberated from the ion exchangers by substitution of the hydrogen and hydroxide ions for the ion components. The liberated ion components move to the ion exchange membrane (anion exchange membrane or cation exchange membrane) through the ion exchanger by electrophoresis, undergo electrodialysis in the ion exchange membrane and move into the concentration chamber. The ion components having moved into the concentration chamber are discharged by water flowing in the concentration chamber.
In the electrodeionization apparatus for producing deionized water, as described above, hydrogen ions and hydroxide ions continuously act as regenerants (an acid and an alkali) for regenerating the ion exchangers. There is, therefore, basically no need to regenerate the ion exchangers with chemicals, and the apparatus can operate continuously.
However, when the deionized water producing apparatus is continuously operated, hardness components in water to be treated precipitate to generate scales, such as calcium carbonate or magnesium hydroxide. Scales are generated particularly on the concentration-chamber-side surface of the anion exchange membrane interposed between the cathode chamber and the concentration chamber (see FIG. 8). In a case where a plurality of deionization chambers are provided, scales are generated on the surface of the anion exchange membrane in the concentration chamber located between two of the deionization chambers (see FIG. 9). This is for the reason described below. Due to passage of hydroxide ions generated by electrolysis in the cathode chamber or hydroxide ions generated by water-splitting reaction in the deionization chamber, the anion exchange membrane surface in the concentration chamber becomes alkaline. The hardness components (magnesium ions or calcium ions) having passed through the cation exchange membrane from the deionization chamber react under the alkaline conditions at the anion exchange membrane surface to generate calcium hydroxide or magnesium hydroxide. If carbonic ions are contained in concentrated water, calcium carbonate or magnesium carbonate is further generated. When scales are generated, the electrical resistance at the area on which scales are generated is increased and a current cannot flow easily therethrough. That is, a need arises to increase the voltage to obtain the same current values as those where there is no scale, resulting in an increase in power consumption. There is also a possibility of nonuniformity of current density in the concentration chamber. If the amount of scale is further increased, the differential pressure for passing water is increased and electrical resistance is further increased. In such a case, current flow, that is required for the removal of ions, fails, which results in the deterioration in the quality of treated water. In addition to this, there is also a possibility of grown scale penetrating to inner part of the ion exchange membrane, leading to damage to the ion exchange membrane.
As a method of preventing the generation of scale described above, filling the concentration chamber with an anion exchanger has been proposed. For example, Patent Literature 1 discloses a deionized water producing apparatus having an anion exchanger of a particular structure disposed on the anion exchange membrane side in a concentration chamber. In this deionized water producing apparatus, the diffusion of OH− into the concentrated water is promoted at the porous anion exchanger surface and rapidly reduces the OH− concentration at the surface. On the other hand, it becomes difficult for hardness component ions to permeate the porous anion exchanger. As a result, the chance of contact and reaction between OH− and hardness component ions is reduced, and precipitation and accumulation of scale are prevented.
Patent Literature 2 discloses a deionized water producing apparatus in which two or more layers of ion exchangers differing in water permeability are provided in a concentration chamber, and the layer of the ion exchanger having lower water permeability is disposed on the anion exchange membrane side, anion exchange groups of which are provided at least in the surface of the layer. In this deionized water producing apparatus, when concentrated water containing a large amount of hardness components moving from a higher water permeability layer reaches the lower water permeability layer, the force of moving the concentrated water is reduced. As a result, the flow of concentrated water containing a large amount of hardness components into the concentration-chamber-side surface of the anion exchange membrane is blocked, thereby preventing precipitation and accumulation of scale.